There are numerous instances in which underground storage tanks have leaked and surrounding soils and groundwater have become contaminated with the contents of the tanks. In some cases a contaminating liquid is spilled on a surface of soil and migrates into the soil. Through these and other mechanisms, there have been many occurrences of soil contamination by liquids which can be generically described as Light, Non-Aqueous Phase Liquids (LNAPLs). When LNAPLs are present in a soil, the region of contamination is referred to as a plume.
In-situ recovery or removal of these LNAPLs from a soil subsurface can be problematic, especially in finer grained materials exhibiting lower permeability. The most common technique for such recovery involves installing a group of wells within the LNAPL plume area. The goal of this technique is to allow the LNAPLs to migrate into the wells across a natural pressure gradient. After the LNAPLs are in the wells, they are extracted and carried away from the contaminated site.
This commonly employed technique is very slow and expensive to implement. Various prior art systems have been devised in an effort to improve the efficacy of a well extraction technique. These efforts have focused on increasing the pressure gradient to which the LNAPLs are subjected so that they migrate more rapidly to the extraction wells.
In one prior art technique, the water table is depressed by pumping. This increases LNAPL recovery by increasing the gradient to the well. However, this technique tends to smear the LNAPL downward, thus increasing the dissolved contaminant levels when the pump is shut down. In addition, disposal of contaminated pumped groundwater becomes a problem and adds to overall treatment costs.
In another prior art technique, vacuum is applied to one or more of the recovery wells. This produces an increase in the gradient and an improved rate of LNAPL collection in the well. In some instances, air injection wells are placed in the plume. This provides for an improved flow of air through the plume and improves the recovery rate. These prior art techniques are described in U.S. Pat. No. 4,593,760 ( Visser et al.), issued Jun. 10, 1986.
When the vacuum assisted recovery technique is employed, there is an effort made to locate a collection area of the well at a depth that is just above the water table height. This is done in an effort to minimize the amount of groundwater that is extracted when the collected LNAPLs are pumped out of the well. However, the presence of vacuum in the well produces a mounding of the groundwater near the well and collection of groundwater along with LNAPLs is inevitable.
In the prior art, LNAPLs are pumped from the extraction wells with skimmer pumps which are designed to minimize the amount of groundwater that is pumped to the surface.
In spite of carefully controlling well depth in relation to water table height and in spite of using sophisticated skimmer pumps, there has heretofore been no way of extracting LNAPLs without extracting some amount of co-mingled groundwater.
When contaminated groundwater is brought to the surface two problems develop. First of all, the groundwater increases the mass of materials which must be carried away from the site and disposed at substantial expense. An even greater problem develops in geographic locations where winter conditions cause freezing of the groundwater that reaches the surface. This freezing requires that remediation operations must be suspended during winter months or that the system must be constructed to allow cold weather operations. These factors add greatly to the cost and overall length of time of a remedial operation.
It is a goal of the present invention to provide a technique for rapidly extracting LNAPLs from contaminated soil without having the LNAPLs co-mingled with groundwater.